An untrained skill check occurs when a character uses a skill in which the character has no ranks. Not all skills allowed untrained checks; those that do not are marked as either "Requires Training: Yes" or "Untrained: No". (NWNWiki uses the former.)
Buying a rank in a skill that requires training can be useful, even in cross-class skills. For example, a character with 16 dexterity and no ranks in open lock is unable to pick a lock. If the same character buys a single rank in open lock, the character is able to attempt to pick locks with a +4 modifier (+1 from the rank and +3 from dexterity), and is able to use Thieves' Tools.